1. Field of the Invention
The present invention relates to a balance filter that includes an unbalanced-side coupling coil and a balanced-side coupling coil and that performs balance-unbalance conversion.
2. Description of the Related Art
For example, Japanese Unexamined Patent Application Publication No. 2011-124880 discloses a laminated balance filter that includes a laminate of a plurality of dielectric layers and electrode layers.
FIG. 13 is an exploded perspective view of the laminated balance filter disclosed in Japanese Unexamined Patent Application Publication No. 2011-124880. FIG. 14 is an equivalent circuit diagram of the laminated balance filter. As shown in FIG. 13, the laminated balance filter is composed of a laminate in which a plurality of dielectric layers 251, 242, 241, 271, 211, 201, and 221 in each of which a predetermined electrode pattern is formed are laminated.
As shown in FIG. 14, the laminated balance filter includes an unbalanced terminal Pu, a ground terminal Pg, and balanced terminals Pb1 and Pb2. A coil Lu in FIG. 14 corresponds to an unbalanced-side coil composed of via electrodes 151 and 152 and an unbalanced-side coil 131 shown in FIG. 13. A coil Lb in FIG. 14 corresponds to a coil 142 in the middle among three balanced-side coils shown in FIG. 13. A coil L1 corresponds to a coil composed of a balanced-side coil 141 and a via electrode 153 shown in FIG. 13. Similarly, a coil L2 corresponds to a coil composed of a balanced-side coil 143 and a via electrode 154 shown in FIG. 13.
In addition, a capacitor Cu in FIG. 14 is a capacitance generated between a capacitor electrode 121 and a ground electrode 120. A capacitor C1 is a capacitance generated between a capacitor electrode 122 and the ground electrode 120. Similarly, a capacitor C2 is a capacitance generated between a capacitor electrode 123 and the ground electrode 120.
The coil Lu and the capacitor Cu shown in FIG. 14 define an LC parallel resonator. A circuit composed of the coils L1 and L2 and the capacitors C1 and C2 serves as a circuit for impedance matching and impedance conversion.
The balance filter disclosed in Japanese Unexamined Patent Application Publication No. 2011-124880 allows the output impedance to be easily adjusted by the coils L1 and L2 and the capacitors C1 and C2 which are connected to the respective balance output terminals. In addition, the balance filter does not use a line of a ½ wave length and thus has an advantage that it is possible to reduce the element size.
In general, balance filters including one in Japanese Unexamined Patent Application Publication No. 2011-124880, an unbalanced-side coupling coil and a balanced-side coupling coil are electromagnetically coupled to each other, whereby signal conversion is performed between the unbalanced side and the balanced side and the degree of coupling between the coupling coils is mainly determined by the distance between the coupling coils. Therefore, when it is impossible to reduce the distance between the coupling coils due to manufacturing limitations or when the distance between the coupling coils is increased due to manufacturing variation, the degree of coupling between the coupling coils is decreased and the insertion loss of the balance filter is increased. In addition, it is effective to increase the winding diameter in order to increase the degree of coupling between the coupling coils. However, for that, the size has to be inevitably increased.